Studies of the pathogenesis of visna in sheep have been focused in two areas. (1) The mechanism of the persistent virus infection and (2) the mechanism of induction of the lesions. While only little progress has been made on the latter aspect, considerable insight has been obtained on the mechanism of persistence of the agent in immune animals. Tissues from inoculated lambs were examined by various parameters for the state of the viral genome in vivo. Tissue suspensions lacked infectivity but large numbers of cell clones derived from the tissue yielded virus. Examination of the tissue sections by in situ hybridization with labelled visna virus DNA indicated that just as large a number of cells which yielded virus had proviral DNA in thier nuclei. This indicated that proviral DNA, possibly in an integrated state was one of the major factors in maintaining visna virus in an immune host. These animals were resistant to disease and in the coming year new breeds of sheep will be tested for susceptibility to disease. This would provide animals in which the mechanism of disease could be investigated from the standpoint of an agent whose genome could integrate in host cells.